1. Field of the Invention
The present invention relates to information storage media possessing optical servo tracks and a method for stamping the optical servo tracks into the media. More particularly, the media is a magnetic disk media, the optical servo tracks comprise a plurality of pits grouped in concentric rings on the surface of the media and the stamping method involves pressing a stamper disk directly onto the magnetic media under pressure thereby transferring the optical servo track pattern onto the magnetic media.
2. Description of the Prior Art
The track density of magnetic storage disks for conventional floppy disk drives is approximately forty-eight to one hundred thirty-five tracks per inch (TPI). In contrast, optical disk drives are capable of achieving track densities in excess of 15,000 TPI. These higher track densities are achieved through the use of closed loop optical servos that allow the read/write head to follow data track eccentricities caused by defects in the medium and by disturbances from outside forces. In rigid type magnetic disk drives, track densities of up to 2100 TPI are presently used. These drives commonly have multiple disks in which both sides are used for data. To achieve the high track density a dedicated surface of one of the disks is used for magnetic track servo information. This surface is then not usable for data storage. Thus, the total capacity of the drive is lessened. The tracking servo information is also capable of being accidentally erased causing loss of access to all data.
Various techniques have been reported for using optical means for acquiring track following servo information contained on a magnetic recording medium. For example, Ahn, et al., in U.S. Pat. No. 4,633,451, issued on Dec. 30, 1986, for "Optical Servo For Magnetic Disks", discloses the use of a laser diode to read track following servo information in the form of a plurality of spots contained in an optical layer positioned above a magnetic recording layer.
M. Johnson, in U.S. Pat. No. 4,558,383, issued on Dec. 10, 1985, for "Information Storage Disk Transducer Position Control System Using a Prerecorded Servo Pattern Requiring No Alignment With the Storage Disk", discloses a servo apparatus having a sensor for detecting a pattern of spots on a surface of an information storage medium. The spots comprise a dense array of substantially translation invariant marks and separate information recording tracks are detected by measuring the rate at which the spots are detected by the sensor.
J. Cocke, et al., in U.S. Pat. No. 4,587,579, issued on May 6, 1986, for "System for Position Detection on a Rotating Disk", discloses a servo control system comprising a detector for reading a plurality of spiral radial-position-encoding patterns on a medium.
H. Kinjo, et al., in U.S. Pat. No. 4,315,283, issued Feb. 9, 1982, discloses an apparatus for burning a servo pattern comprising a plurality of oval pits into the surface of a medium.
A. S. Hoagland, in "Optical Servo of Magnetic Recording", IBM Technical Disclosure Bulletin, Vol. 20, No. 10, page 4108 (March 1978), suggests a system for achieving optical servo control where a flexible disk medium includes a plurality of optical servo tracks positioned underneath a magnetic layer.
D. A. Thompson, et al., in "Embossed Servo Techniques For Floppy Disks", IERE Conference Proceedings, No. 43, p. 321 (July 1979), discloses the use of embossed marks on magnetic tape media, and dies for making the embossed marks, for obtaining non-magnetic optical or capacitive servo information. The paper suggest that the technique could be applied to floppy disks.
N. Koshino and S. Ogawa, in "Optical Method of the Head Positioning in Magnetic Disk Systems", preprint from IEEE Transactions on Magnetics (1980), discloses an optical head for achieving track following servo control which is mounted on the head arm and which includes an LED light source and three optical fibers for delivering light to a medium. The medium comprises a plurality of circular optical tracks, dyed black, and located underneath a magnetic film.
Related work has occurred in the laser video disk area, from which optical disks for digital data storage and the audio laser disk (CD) have evolved. Fundamentally, the optical servo information is inscribed and used in the same way for all these disks. A mastering machine is used to format optical information onto a master disk. The master is then replicated to form the actual disk used by the customer. A laser and associated optics are used to acquire the mastered servo information as well as read data from the disk. The data can be inscribed during the mastering process as with the video and audio disks or it can be written by the read/write laser as in disks for digital information storage.
R. E. Acosta, et al., in "Floppy Disc Embossing For Servo Applications", IBM Technical Disclosure Bulletin, Vol. 21, No. 10 (March 1979), suggests adopting read-only video technology for the servo control of floppy disks. It is suggested that a master die could be used to stamp or emboss a pattern of indentations for servo tracking purposes on a magnetic recording medium. However, no actual stamping method or servo pattern is disclosed.
K. D. Broadbent, in "A Review of the MCA Disco-Vision System", Journal of the SMPTE (1974), describes the laser video mastering technique as well as the servo and read back methods. A master disk is formed by using an argon laser to ablate pits in a metallic layer deposited on a glass plate. The master disk is then coated with photoresist which is exposed through the pits. After washing away the undeveloped photoresist, "bumps" of polymerized photoresist are left over the pits. This paper refers to, but does not disclose, a process for using the master disk to form replicas on a polyethylene terephthalate (Mylar) medium.
Broadbent's paper also states that the master disk can be electrolytically treated to form a stamper tool from which replicas are thermoformed.
J. S. Winslow, in "Mastering and Replication of Reflective Videodiscs", IEEE Transactions on Consumer Electronics, p. 318, November 1976, describes the videodisk mastering technique in further detail.
Similarly, B. Jacobs, in "Laser Beam Recording of Video Master Disks", Applied Optics, Vol. 17, p. 2001 (July 1, 1978), describes the process of making a master disk using a laser to inscribe a pit pattern on a rotating substrate coated with a thin film of photoresist.
An article in OEP, p. 48, May 1987, entitled "Daicel Leads Development of the Optical Memory Disk Supported by Chemical Technology", shows how a stamper disk is used to form an optical disk through an injection molding process.
None of these references disclose a practical method for applying high track density optical servo tracks to a floppy disk type magnetic medium.